Commonly used current detection circuits for detecting the current flowing through a load controlled by a transistor employs a current detection resistor connected in series to the transistor or to the load to directly detect the current by measuring the voltage drop across the resistor, as disclosed in Japanese Patent Application Laid Open H11-299292 (hereinafter referred to as Patent Document 1) and 2003-174766 (referred to as Patent Document 2).
Another type of current detection circuit is also disclosed in, for example, Japanese Patent Application Laid Open No. 2570523 (referred to as Patent Document 3), in which constant current is passed through a current detection transistor while applying thereto the same control voltage as applied to the transistor connected to the load, and the output voltages of the respective transistors are compared to detect the level of the load current.
The conventional current detection circuits as disclosed in Patent Documents 1 and 2 inevitably suffers from (PWM) energy loss in the current detection resistors used, which lowers the power efficiency of the circuit. In addition, in a load drive circuit having a bridge circuit adapted to perform pulse-width modulated (PWM) driving of a load, the current detection circuit cannot perform current detection per se during off periods of the pulse width modulation (PWM).
Although the current detection circuit of Patent Document 3 is free of power loss by a current detection resistor, it cannot detect the load current continuously, since the circuit is intended to determine if the load current exceeds a predetermined level or not.
In view of such prior art problems, the present invention is directed to a high-precision current detection circuit capable of continuously detecting current in a stable manner with reduced power loss in the current detection. The invention is also directed to a load drive circuit utilizing such current detection circuit.